1. Field of the Invention
The invention relates to new coating compositions containing a one- or two-component binder and an adhesion promoter, to a process for coating plastic substrates and to the resulting coated plastic substrates.
2. Description of the Prior Art
For the application of strongly adhering lacquer coatings to plastics, especially to PP (polypropylene) injection molded components, a chemical or physical modification of the substrate surfaces is necessary. Known modifications include flaming (for example F. Carbassi et al., J. Mater. Sci., 22, pages 207 to 212 (1987)), corona treatment (for example F. Lawson, Rubber Chem. and Technol., 60, 102 (1987)), plasma treatment (for example L. H. Coopes et al., J. Macromol. Chem. 17, pages 217 to 226 (1982) and EPA 0 282 094), and etching with chromosulphuric acid (for example M. Salkauskas, J. Appl. Polym. Sci., 26, pages 2079 to 2098 (1982)).
It is also disclosed that it is possible by the use of adhesion promoter coatings based on chlorinated polyolefins (U.S. Pat. No. 3,676,391) or maleic anhydride-modified polyolefins, especially amorphous polypropylene types (DE-OS 1,546,982) to improve the adhesion of coating to plastic components based on polypropylene and polyethylene.
These known methods are either costly or, in the case of injection molded components with complicated surfaces, inapplicable. In addition to the high treatment costs, the application of these adhesion promoter coatings may also result in environmental problems due to their low solids content. Moreover its application to particular lacquer systems is limited.
In PCT Application number WO 88/05346 it is proposed to modify the adhesion promoter coatings by means of additional measures such as UV irradiation, which leads to an additional expense.
The addition of chlorinated polyolefins, for example, the conventional chlorinated polypropylenes recommended in U.S. Pat. No. 2,468,480, to the coating compositions also has the disadvantage that such chlorinated polyolefins are not very compatible with the known plastics coating compositions. Furthermore, their solubility in the typical coating solvents is very small, which negatively affects the application of the coating compositions.
In DE-OS 1,546,982 it is recommended to use chemically modified amorphous polyolefins as a primer. However, it is not possible to directly add these polyolefins to coating compositions as adhesion promoters due to their incompatibility with the binders of these compositions.
In DE-OS 3,909,217 and 3,939,760 it is recommended to use coating compositions that contain at least one adhesion promoter based on chlorine-containing, chemically modified amorphous polyolefins with functional groups for the direct coating of plastic injection-molded components. For ecological reasons, however, chlorine-free systems are becoming increasingly important.
It is therefore an object of the present invention to provide a new coating composition for the direct, adherent coating of plastics which is suitable, due to the use of novel, chlorine-free adhesion promoters, for the production of high-grade coatings; which does not suffer from the disadvantages of the prior art coating compositions; and wherein the adhesion promoters can be incorporated into the coating compositions without sedimentation.
These objects may be achieved with the coating compositions of the present invention which contain certain chemically modified polyolefins as adhesion promoters and which are described in more detail hereinafter.